A conventional supervisory system is shown in FIG. 1 in which a variety of Field equipments 2 are connected to a host 1 through a low speed serial port (e.g., RS-232 or RS-485). Multiple device drivers are installed in the host 1 to drive field equipments 2. The host 1 and field equipments 2 are a client-server architecture and the host 1 is always polling and reading data from Field equipments 2. However, the prior art shown in FIG. 1 suffers from the use of low speed rate and short communication distance serial port which limits the data response time and work range of the application system. The need for multiple device drivers in the host 1 also limits the expansion of the application system. Another solution that involves a conventional supervisory system is shown in FIG. 2, in which a variety of field equipments 5 are connected to the host 3 through network gateways 4, which are used as communication protocol adaptor between the serial port (e.g., RS-232 or RS-485) and the Ethernet port. The host 3 thinks network gateways 4 acting as virtual serial ports. Similarly, It suffers from the same problem as the prior art shown in FIG. 1, except that the work range of the application system is improved due to the use of Ethernet.